This study is a double blind randomized clinical trial prevention of basal cell epithelioma and squamous cell of carcinoma of the skin with a nutritional supplement of selenium. The treatment is 200 mcg of selenium as a selenium enriched brewer's yeast, the placebo is an identical tablet of brewer's yeast. This dose and form of Se has been approved for use in this trial by the food and drug Administration since it is a commercially available product. In the study which preceeds this application over 241 patients have been randomized with no observed toxic effects from treatment. The treatment group has shown a good response to the selenium supplement with their initial plasma levels of Se rising from .12 mcg/ml at the 6 month and at the 12 month examination. This clinical trial has been preceded by a case control and cohort study which demonstrated an increase risk of non-melanoma skin cancer (NMSC) among patients with low plasma selenium levels. These two epidemiologic studies and the current trial provide the justification and experience necessary to conduct this trial. Based on these two epidemiologic studies we predict that a .1 mcg/ml rise in plasma Se levels should be associated with a 50% reduction in the ratio of time to new NMSC if the patients are followed for a sufficient, but unknown period of time. This study is designed to detect a 26% reduction in the ratio of time to new NMSC during the minimum 3.5 years of treatment and follow-up. This study will provide the critical test of the hypothesis that improving selenium status can reduce the risk of cancer.